We are studying the expression of mRNAs that are abundant in the mouse liver. Our objective is to analyze transcription sites of these sequences in isolated nuclei, to determine the timing of accumulation of the sequences during development and to determine the frequency of the genes coding for these mRNAs as it has been shown previously that some elements in the abundant mRNAs are repeated in the genome. During the past year, we have purified cDNA fragments corresponding to an mRNA that is relatively abundant in the kidney (KAP mRNA) and to an mRNA that is abundant in the liver (MUP mRNA). Both mRNAs are androgen regulated. The KAP mRNA is expressed only in the kidney. MUP mRNA is expressed in the liver, kidney and submaxillary gland at different levels in each but appears to be absent from the testes, brain, and spleen. By hybridization analysis we have shown that several of the abundant liver mRNAs are expressed at very low concentration if at all in the 12.5 day foetal liver. The MUP mRNA is not present in the livers of newborn or one week old mice. We have also shown that there are 10-15 copies of the gene for MUP in liver DNA. During the next year, we intend to purify cDNA fragments corresponding to other abundant liver mRNAs. Also we will determine the timing of onset of MUP gene expression in the liver and kidney. We will analyze the DNAase 1 sensitivity of the MUP genes in chromatin to ascertain how many of the genes are transcribed. Also we will determine the timing of onset of DNAase 1 sensitivity relative to the onset of MUP mRNA expression. Also, physical methods will be employed to try to enrich MUP genes. Eventually we should like to ascertain whether the multiple genes are clustered. Finally, we will develop an in vitro transcription system using nuclei.